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Abstract:

In an optical scanning apparatus, an aperture is provided between a
semiconductor laser in a light source unit and an oscillating mirror, and
between a cylindrical lens and the oscillating mirror. When a light beam
from the semiconductor laser comes into an reflection surface of the
oscillating mirror, the optical scanning apparatus is configured to limit
a beam width of the light beam to a width appropriate to the reflection
surface, and then to ensure an irradiation position of the light beam in
a main-scanning direction to come into the reflection surface of the
oscillating mirror, by causing the light beam to pass through an opening
of the aperture.

Claims:

1. An optical scanning apparatus comprising:a light source unit that emits
a light beam;a light-source activating unit that activates the light
source unit;an aperture unit having an opening for limiting a beam width
of the light beam output from the light source unit;a light deflection
unit that includes a reflection surface configured to deflect the light
beam output from the light source unit, the reflection surface being
configured to rotate freely about a twist beam; andan optical system that
forms an image into a spot onto a scan target surface with the light beam
deflected by the light deflection unit,wherein the aperture unit is
arranged such that a center of the light beam incident to the light
deflection unit substantially matches a rotation axis of the reflection
surface.

2. The optical scanning apparatus according to claim 1, further comprising
a linear-image forming lens that forms a linear image by converging the
light beam output from the light source unit only into one direction,
wherein the aperture unit is arranged between the linear-image forming
lens and the light deflection unit.

3. The optical scanning apparatus according to claim 1, further comprising
a transparent member through which the light beam deflected by the light
deflection unit passes,wherein the transparent member includes an
aperture unit having an opening for limiting a beam width of the light
beam output from the light source unit.

4. The optical scanning apparatus according to claim 1, further comprising
a light-amount adjusting unit that adjusts an amount of light of the
light beam deflected by the light deflection unit along the main-scanning
direction.

5. The optical scanning apparatus according to claim 4, wherein the
light-amount adjusting unit increases an amount of passing-through light
from an incidence side of the light beam to a side opposite to the
incidence side with respect to a light axis of at least one scan lens of
the scan imaging optical-system in the main-scanning direction.

6. The optical scanning apparatus according to claim 3, wherein the
light-amount adjusting unit is provided in the transparent member.

7. The optical scanning apparatus according to claim 4, wherein the
light-amount adjusting unit is provided in the transparent member.

8. The optical scanning apparatus according to claim 1, wherein the
light-source activating unit adjusts a pulse width along the
main-scanning direction to increase an amount of light from an incidence
side of the light beam to a side opposite to the incidence side.

9. The optical scanning apparatus according to claim 1, wherein the
light-source activating unit adjusts a light beam intensity along the
main-scanning direction to increase an amount of light from an incidence
side of the light beam to a side opposite to the incidence side.

10. The optical scanning apparatus according to claim 1, further
comprising:a light-amount detecting unit that detects an amount of light
of the light beam deflected by the light deflection unit; anda
light-amount adjusting unit that adjusts an amount of light of the light
source unit, wherein a reference value for the light-amount adjusting
unit is set based on a signal obtained by the light-amount detecting
unit.

11. An image forming apparatus comprising an image carrier, a charging
device, a developing device, and the optical scanning apparatus according
to claim 1 as an optical scanning device.

12. An optical scanning apparatus comprising:a light source unit that
emits a light beam;a light deflection unit that includes a reflection
surface configured to deflect the light beam output from the light source
unit, the reflection surface being configured to rotate freely about a
twist beam; andan optical system that forms an image into a spot onto a
scan target surface with the light beam deflected by the light deflection
unit,wherein a beam width of the light beam is limited by the reflection
surface at least in a main-scanning direction.

13. The optical scanning apparatus according to claim 12, wherein edges of
the reflection surface in the main-scanning direction are substantially
straight in a sub-scanning direction.

14. The optical scanning apparatus according to claim 12, wherein at least
a width of the reflection surface in the main-scanning direction is
smaller than a width of the light deflection unit in the main-scanning
direction.

15. The optical scanning apparatus according to claim 12, further
comprising a transparent member through which the light beam deflected by
the light deflection unit passes,wherein the transparent member includes
an aperture unit having an opening for limiting a beam width of the light
beam output from the light source unit.

16. The optical scanning apparatus according to claim 12, further
comprising a light-amount adjusting unit that adjusts an amount of light
of the light beam deflected by the light deflection unit along the
main-scanning direction.

17. The optical scanning apparatus according to claim 16, wherein the
light-amount adjusting unit increases an amount of passing-through light
from an incidence side of the light beam to a side opposite to the
incidence side with respect to a light axis of at least one scan lens of
the scan imaging optical-system in the main-scanning direction.

18. The optical scanning apparatus according to claim 15, wherein the
light-amount adjusting unit is provided in the transparent member.

19. The optical scanning apparatus according to claim 16, wherein the
light-amount adjusting unit is provided in the transparent member.

20. An image forming apparatus comprising an image carrier, a charging
device, a developing device, and the optical scanning apparatus according
to claim 12 as an optical scanning device.

Description:

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001]The present application claims priority to and incorporates by
reference the entire contents of Japanese priority document 2007-057659
filed in Japan on Mar. 7, 2007.

BACKGROUND OF THE INVENTION

[0002]1. Field of the Invention

[0003]The present invention relates to an optical scanning apparatus for
use in an image forming apparatus.

[0004]2. Description of the Related Art

[0005]To scan with a light beam onto a photosensitive element, various
optical scanning apparatuses are used in image forming apparatuses, such
as, a digital photocopier, a facsimile, and a laser printer. According to
an optical scanning apparatus that has been conventionally used, a
polygon mirror or a galvanometer mirror have been used as a deflector
that deflects a light beam from a light source.

[0006]However, to form an image in a higher resolution for a shorter time,
it needs to rotate such polygon mirror or galvanometer mirror at a higher
speed. There is a limitation to rotate the polygon mirror or the
galvanometer mirror at a higher speed due to obstacles, such as noise,
heat during rotation, and an endurance of a bearing that rotatably
supports the polygon mirror or the galvanometer mirror.

[0007]For this reason, as a deflector used in the optical scanning
apparatus, a deflector produced by silicon micro machining is recently
proposed (for example, see Japanese Patent Publication No. 2924200,
Japanese Patent Publication No. 3011144, and Japanese Patent Application
Laid-open No. 2002-82303).

[0008]As shown in FIG. 20, a deflector 501 in this type has an integrally
molded structure formed of an oscillating mirror 502 and twist beams 503,
the surface of the oscillating mirror 502 forming a reflection surface
502a, and the twist beams 503 supporting the oscillating mirror 502 as a
pivot. The deflector 501 has advantages that a small size can be achieved
by making the oscillating mirror 502 small in size, and that the
deflector 501 works with a low noise and at a low power consumption in
spite of that high speed operation is available, because the oscillating
mirror 502 is reciprocated and oscillated by using resonance of the
oscillating mirror 502.

[0009]Furthermore, the deflector 501 has another advantage that because
the deflector 501 causes low oscillation and almost no heat, a housing to
accommodate the optical scanning apparatus can be made of thin walls, so
that the housing is constructed with a resin molding material at low cost
that contains glass fiber at a low mix proportion, still the image
quality is hardly influenced.

[0010]Particularly, Japanese Patent Application Laid-Open No. 2002-82303
discloses an example that the deflector 501 is used instead of a polygon
mirror. The example proposed is an image forming apparatus that is
suitable for office environment and appropriate to the global environment
because low noise and low power consumption are achieved by using an
oscillating mirror as a substitute for a polygon mirror.

[0011]However, when the oscillating mirror 502 is driven, dynamic surface
deformation due to a moment of inertia and a restoring force of the
oscillating mirror 502 occurs as described below.

[0012]Suppose dimensions of the oscillating mirror 502 shown in FIG. 20
are 2a in the longitudinal direction, 2b in the transverse direction, d
in thickness, and the density of silicon is ρ. The moment of inertia
I of the oscillating mirror 502 is expressed in the following equation 1.

Moment of inertia I=(4abpd/3)×a2 (1)

[0013]As shown in equation 1, the moment of inertia I of the oscillating
mirror 502, which is a local moment, is a function of a distance from the
rotation axis of the oscillating mirror 502, and the longer distance from
the rotation axis leads to the larger moment of inertia.

[0014]The thickness of the oscillating mirror 502 is a few hundreds
micrometers, which is thin, so that coming up with change in the rotation
speed due to reciprocating oscillation and the moment of inertia applied
on the oscillating mirror 502, a force is exerted in opposite directions
at a point in the vicinity of the twist beam 503 of the oscillating
mirror 502 and an end away from the twist beam 503, consequently the
oscillation mirror 502 is waved and deformed as shown in FIG. 21.

[0015]Accordingly, a wavefront aberration of the light flux of the light
beam reflected by the oscillating mirror 502 becomes large, so that the
light beam becomes thick.

[0016]FIG. 21 depicts a state of deformation of the oscillating mirror 502
formed as a simple plate. Along with degradation of the wavefront
aberration of the light flux, as indicated by dashed lines shown in FIG.
21, deviations of incidence positions are produced in the direction
orthogonal to the twist beams 503 (main-scanning direction).

[0017]In such case, apparent curvatures are different, so that imaging
positions of the light beam are deviated (out of focus). Particularly due
to an assembling deviation of the deflector or the light source, when the
light beam is irradiated to an edge of the oscillating mirror 502 as
shown in FIGS. 22A and 22B, the light beam at an imaging position 505
becomes thick, or out of focus.

[0018]Consequently, the light beam irradiated to the edge of the
oscillating mirror 502 becomes a converging light flux in the
main-scanning direction (see FIG. 22A), or a diverging light flux (see
FIG. 22B), as a result, the light beam cannot be uniformly converged onto
the imaging position 505, so that a desired beam-spot diameter cannot be
obtained.

[0019]For this reason, conventionally the light beam cannot be converged
across an entire scanned surface, the beam-spot diameter cannot be kept
constant, resulting in a problem of degradation of the image.

SUMMARY OF THE INVENTION

[0020]It is an object of the present invention to at least partially solve
the problems in the conventional technology.

[0021]According to an aspect of the present invention, there is provided
an optical scanning apparatus including a light source unit that emits a
light beam; a light-source activating unit that activates the light
source unit; an aperture unit having an opening for limiting a beam width
of the light beam output from the light source unit; a light deflection
unit that includes a reflection surface configured to deflect the light
beam output from the light source unit, the reflection surface being
configured to rotate freely about a twist beam; and an optical system
that forms an image into a spot onto a scan target surface with the light
beam deflected by the light deflection unit. The aperture unit is
arranged such that a center of the light beam incident to the light
deflection unit substantially matches a rotation axis of the reflection
surface.

[0022]According to another aspect of the present invention, there is
provided an optical scanning apparatus including a light source unit that
emits a light beam; a light deflection unit that includes a reflection
surface configured to deflect the light beam output from the light source
unit, the reflection surface being configured to rotate freely about a
twist beam; and an optical system that forms an image into a spot onto a
scan target surface with the light beam deflected by the light deflection
unit. A beam width of the light beam is limited by the reflection surface
at least in a main-scanning direction.

[0023]The above and other objects, features, advantages and technical and
industrial significance of this invention will be better understood by
reading the following detailed description of presently preferred
embodiments of the invention, when considered in connection with the
accompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

[0024]FIG. 1 is a schematic diagram of an inside front of an image forming
apparatus according to a first embodiment of the present invention;

[0025]FIG. 2 is a schematic diagram for explaining relevant parts in an
image forming apparatus shown in FIG. 1, such as a laser writing unit as
an optical scanning apparatus and a photosensitive element;

[0026]FIG. 3 is an exploded perspective view of the laser writing unit in
the image forming apparatus shown in FIG. 1;

[0027]FIG. 4 is a schematic diagram for explaining relevant parts in the
image forming apparatus shown in FIG. 1, such as the laser writing unit
and the photosensitive element;

[0028]FIG. 5 is an exploded perspective view of a light source device in
the laser writing unit shown in FIG. 3;

[0029]FIG. 6 is an exploded perspective view of a deflection unit in the
light source device shown in FIG. 5;

[0030]FIG. 7A is a front view of an oscillating mirror in the deflection
unit shown in FIG. 6;

[0031]FIG. 7B is a back view of a mirror unit of the oscillating mirror
shown in FIG. 6;

[0032]FIG. 7C is a cross sectional view of the mirror unit shown in FIG.
7B along a line between VIC and VIC;

[0033]FIG. 8 is an exploded perspective view of the oscillating mirror
shown in FIG. 7A;

[0034]FIG. 9A is an exploded perspective view of a light source unit in
the light source device shown in FIG. 5;

[0035]FIG. 9B is an exploded perspective view from the back side of the
light source unit shown in FIG. 9A;

[0036]FIG. 10A is a schematic diagram for explaining distributions of
light intensities before and after passing through an aperture when the
position of the light source unit is deviated in the light source device
shown in FIG. 5;

[0037]FIG. 10B is a schematic diagram for explaining distributions of
light intensities according to a conventional auto power control before
and after passing through the aperture when the light source position is
deviated;

[0038]FIGS. 11A and 11B are schematic diagrams for explaining a function
of limiting a beam width by changing a positional relation between a
reflection surface of the oscillating mirror and the aperture shown in
FIG. 4;

[0039]FIG. 12 is a schematic diagram for explaining a deflection unit
includes a component that combines a transparent member and an opening;

[0040]FIG. 13 is a schematic diagram for explaining a relation between the
width of a reflection surface of an oscillating mirror and a function of
limiting a beam width by an aperture according to a second embodiment of
the present invention;

[0041]FIGS. 14A and 14B depict shapes of reflection surfaces according to
the second embodiment;

[0042]FIGS. 15A and 15B are schematic diagrams for explaining diameters of
incident beams according to the second embodiment;

[0043]FIG. 16 is a graph that represents a shading property of a
transparent member according to the second embodiment;

[0044]FIG. 17 is a schematic diagram for explaining the shading property
of the transparent member according to the second embodiment;

[0045]FIGS. 18A and 18B are schematic diagrams for explaining adjustment
of the amount of light according to a third embodiment of the present
invention;

[0046]FIG. 19 is a schematic diagram for explaining an optical scanning
apparatus according to a fourth embodiment of the present invention;

[0047]FIG. 20 is a schematic diagram for explaining a moving part of a
conventional oscillating mirror;

[0048]FIG. 21 is a schematic diagram for explaining a state that the
moving part of the oscillating mirror shown in FIG. 19 is waved;

[0049]FIG. 22A is a schematic diagram for explaining an example of a
deflection of a light beam incident to a concave end of the moving part
in the waved state shown in FIG. 21; and

[0050]FIG. 22B is a schematic diagram for explaining an example of a
deflection of a light beam incident to a convex end of the moving part in
the waved state shown in FIG. 21.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

[0051]Exemplary embodiments of the present invention will be explained
below in detail with reference to the accompanying drawings. First of
all, a first embodiment of the present invention is explained below with
reference to FIGS. 1 to 12.

[0053]The apparatus body 2 can be formed, for example, in a box shape, and
placed on a floor. The apparatus body 2 accommodates the paper feeding
unit 3, the registering roller pair 10, the transferring unit 4, the
fixing unit 5, the laser writing unit 22, and the process cartridge 6.

[0054]The paper feeding unit 3 is placed below the apparatus body 2, and
includes a plurality of paper feeding cassettes 23 and 24 that can be
inserted into and removed from the apparatus body 2 as required. The
paper feeding cassettes 23 and 24 accommodate the paper 7 in a superposed
manner, and the paper feeding cassettes 23 and 24 is provided with paper
feeding rollers 25 and 26, respectively. Each of the paper feeding
rollers 25 and 26 is pressed against a top sheet of the paper 7 in each
of the paper feeding cassettes 23 and 24. The paper feeding rollers 25
and 26 send out the top sheet of the paper 7 towards (a nip) between the
registering roller pair 10.

[0055]The registering roller pair 10 includes a pair of rollers, and is
arranged on a delivery route of the paper 7 to be delivered to the
transferring unit 4 from the paper feeding unit 3. The registering roller
pair 10 holds the paper 7 between the pair of the rollers, and sends out
the paper 7 into between the transferring unit 4 and the process
cartridge 6 in accordance with the timing of superposing toner images
(the timing of the start of recording in a sub-scanning direction
(vertical direction in FIG. 1)).

[0056]The transferring unit 4 is arranged above the paper feeding unit 3.
The transferring unit 4 includes a plurality of rollers 27 and a
transferring belt 29. Each of the rollers 27 is rotatably placed in the
apparatus body 2, and at lease one of the rollers 27 is driven and
rotated, for example, by a motor as a drive.

[0057]The transferring belt 29 is formed into an endless loop, and
threaded around the rollers 27. As threaded around the rollers 27, the
transferring belt 29 is positioned below and in the vicinity of the
process cartridge 6. As at least one of the rollers 27 is driven and
rotated by a motor, the transferring belt 29 is revolved (endlessly runs)
around the rollers 27.

[0058]As the transferring belt 29 presses the paper 7 sent out from the
paper feeding unit 3 onto the outer surface of a photosensitive drum 8 of
the process cartridge 6, the transferring unit 4 transfers a toner image
on the photosensitive drum 8 to the paper 7. The transferring unit 4
sends out the paper 7, on which the toner image is transferred, towards
the fixing unit 5.

[0059]The fixing unit 5 includes a pair of rollers 5a and 5b, which hold
therebetween the paper 7. The fixing unit 5 fixes the toner image on the
paper 7 transferred from the photosensitive drum 8 by pressing and
heating in between the pair of the rollers 5a and 5b the paper 7 sent out
from the transferring unit 4.

[0060]The laser writing unit 22 is arranged in the upper part of the
apparatus body 2, i.e., above the paper feeding unit 3. The laser writing
unit 22 forms an electrostatic latent image by irradiating a laser light
onto the outer surface of the photosensitive drum 8 uniformly charged by
an electrostatic charger 9 of the process cartridge 6. The laser writing
unit 22 performs image recording (forms an electrostatic latent image) of
two line each on the outer surface of the photosensitive drum 8 by
performing a cycle of reciprocation scanning with an oscillating mirror
85. A detailed configuration of the laser writing unit 22 will be
explained later.

[0061]The process cartridge 6 is arranged between the transferring unit 4
and the laser writing unit 22, and is detachable to the apparatus body 2.
As shown in FIG. 2, the process cartridge 6 includes a cartridge case 11,
the electrostatic charger 9 as a charging device, the photosensitive drum
8 as an image carrier, a cleaning case 12 as a cleaning device, and a
development device 13. Consequently, the image forming apparatus 1
includes at least the electrostatic charger 9, the photosensitive drum 8,
the cleaning case 12, and the development device 13.

[0062]The cartridge case 11 is detachable to the apparatus body 2, and
accommodates the electrostatic charger 9, the photosensitive drum 8, the
cleaning case 12, and the development device 13. The electrostatic
charger 9 charges uniformly over the outer surface of the photosensitive
drum 8. The photosensitive drum 8 is arranged apart from a development
roller 15 of the development device 13 keeping a space. The
photosensitive drum 8 is formed into a column or a cylindrical shape
rotatably around the axis of the drum.

[0063]The photosensitive drum 8 is configured to have an electrostatic
latent image formed thereon by the laser writing unit 22. The
photosensitive drum 8 develops thereon a toner image as toner is absorbed
onto the electrostatic latent image formed and carried on the outer
surface of the photosensitive drum 8, and transfers the toner image
obtained in this way to the paper 7 positioned between the transferring
belt 29 and the photosensitive drum 8. The outer surface of the
photosensitive drum 8 is configured to become a surface to be scanned.
The cleaning case 12 removes transfer residual toner remaining on the
outer surface of the photosensitive drum 8 after a toner image is
transferred to the paper 7.

[0064]The development device 13 includes at least a toner cartridge 17,
and the development roller 15 as a developer carrier. The development
device 13 stirs toner in the toner cartridge 17 sufficiently, and absorbs
the stirred toner onto the outer surface of the development roller 15.
The development device 13 then makes the photosensitive drum 8 absorb the
toner by rotating the development roller 15. In this way, the development
device 13 conveys the toner to a development zone by carrying the toner
with the development roller 15, develops an electrostatic latent image on
the photosensitive drum 8, and then forms a toner image.

[0065]The development roller 15 is arranged in parallel with and in the
vicinity of the photosensitive drum 8. The space between the development
roller 15 and the photosensitive drum 8 forms the development zone for
the photosensitive drum 8 to absorb toner and to obtain a toner image by
developing an electrostatic latent image.

[0066]The delivery unit 16 includes delivery trays 18 and 19 arranged on
the upper surface of the apparatus body 2, and pairs of delivery rollers
20 and 21, which are arranged for the delivery trays 18 and 19,
respectively. Each of the pairs of the delivery rollers 20 and 21 is
supplied therebetween with the paper 7 on which a toner image is fixed by
being held between the pair of the rollers 5a and 5b of the fixing unit
5. The pairs of the delivery rollers 20 and 21 deliver the paper 7 on
which the toner image is fixed onto the delivery trays 18 and 19,
respectively.

[0067]The image forming apparatus 1 forms an image on the paper 7 as
described below.

[0068]To begin with, the image forming apparatus 1 uniformly charges the
outer surface of the photosensitive drum 8 with the electrostatic charger
9 by rotating the photosensitive drum 8. By irradiating a laser light to
the outer surface of the photosensitive drum 8, an electrostatic latent
image is formed on the outer surface of the photosensitive drum 8.

[0069]When the electrostatic latent image is positioned in the development
zone, the toner absorbed on the outer surface of the development roller
15 of the development device 13 is absorbed onto the outer surface of the
photosensitive drum 8, the electrostatic latent image is developed, and
then a toner image is formed on the outer surface of the photosensitive
drum 8.

[0070]The image forming apparatus 1 positions the paper 7 conveyed by the
paper feeding rollers 25 and 26 of the paper feeding unit 3 into between
the photosensitive drum 8 of the process cartridge 6 and the transferring
belt 29 of the transferring unit 4, and transfers the toner image formed
on the outer surface of the photosensitive drum 8 to the paper 7.

[0071]The image forming apparatus 1 fixes the toner image onto the paper 7
with the fixing unit 5, and delivers the paper 7 one of the delivery
trays 18 and 19 of the delivery unit 16. Thus, the image forming
apparatus 1 forms an image on the paper 7.

[0072]Details of the laser writing unit 22 are explained below. The laser
writing unit 22, which scans the photosensitive drum 8, having an
integrated configuration as shown in FIG. 2, simultaneously forms an
electrostatic latent image onto the photosensitive drum 8 in accordance
with a moving direction K of the paper 7 (indicated by an arrow shown in
FIG. 2), by deflecting and guiding a light beam from a semiconductor
laser 51 with the oscillating mirror 85. Hereinafter, a direction in
parallel with the axis of the photosensitive drum 8 is denoted by an
arrow X in the drawings, and referred to as a main-scanning direction, a
direction in parallel with the light axis of a light beam deflected by
the oscillating mirror 85 is denoted by an arrow Y and referred to as a
light-axis direction, and a direction orthogonal to both of the
main-scanning direction X and the light-axis direction Y is denoted by an
arrow Z and referred to as a sub-scanning direction.

[0073]As shown in FIGS. 3 and 4, the laser writing unit 22 includes a unit
body 30, a light source device 31, and an imaging optical system 32. As
shown in FIG. 3, the unit body 30 includes three of plate members 34,
which are formed as band plates. The plate members 34 are attached to the
apparatus body 2 to form a square U-shape in a plan view by fastening
edges of the plate members 34 to each other.

[0074]As shown in FIGS. 3, 4, and 5, the light source device 31 includes
an optical housing 35, a light source unit 48, a cylindrical lens 38 as a
linear-image forming lens, a deflection unit 39, and an aperture 130 that
sets the aperture.

[0075]The optical housing 35 includes a housing case 40 and an upper cover
41 of a flat plate, both of which are made from a synthetic resin. The
housing case 40 includes a bottom plate 42 of a flat plate, a plurality
of side plates 43 that is arranged to stand from the outer edges of the
bottom plate 42, and a partition plate 44 in an integrated manner. Two of
the side plates 43 continuous to each other are provided with a fitting
hole 45 configured to mount the light source unit 48, and an emission
window 46. The fitting hole 45 is formed into a circle. The emission
window 46 is formed into a flat rectangle.

[0076]The partition plate 44 partitions the inside of the housing case 40,
i.e., a space inside the optical housing 35, into a space for
accommodating the deflection unit 39, and a space for accommodating items
other than the deflection unit 39.

[0077]The partition plate 44 is provided with a window of a transparent
member 47 in rectangle. The upper cover 41 is attached to the housing
case 40 to close an upper opening formed at edges of the side plates 43
of the housing case 40 on the side apart from the bottom plate 42, and
seals the optical housing 35.

[0078]As shown in FIGS. 9A and 9B, the light source unit 48 includes a
printing substrate 50, the semiconductor laser 51 as a light source unit,
a holder member 53, a coupling lens 54, and a light-source activating
unit, not shown, which activates the semiconductor laser 51. The printing
substrate 50 includes, for example, an insulative substrate, and a wiring
pattern formed on the outer surface of the substrate.

[0079]The semiconductor laser 51 is mounted on the printing substrate 50.
Precisely, the light source unit 48 includes the semiconductor laser 51
as a light source of the process cartridge 6. The semiconductor laser 51
emits a light beam 59 to the photosensitive drum 8.

[0080]The holder member 53 includes a holder body 63 of a thick flat
plate, a pair of supports 64, a laser positioning hole 65, a pair of
projections 66, and a pair of attachment planes 68. The holder body 63 is
provided with spindles 70 projecting and extending along the sub-scanning
direction Z outwardly from the both ends of the holder body 63 in the
sub-scanning direction Z.

[0081]The pair of the supports 64 is arranged at positions on edges of the
holder body 63 opposing to each other with respect to the center of the
holder body 63, and stands from the holder body 63 towards the printing
substrate 50. As the supports 64 are placed to fit to the printing
substrate 50, and screws coming through the printing substrate 50 are
screwed into the supports 64, the supports 64 secure the holder member 53
to the printing substrate 50.

[0083]Each of the attachment planes 68 is formed into a flat plate and
continued to each of the spindles 70. The surfaces of the attachment
planes 68 are substantially flush with the outer surface of the holder
body 63.

[0084]The pair of the projections 66 is formed from the holder body 63 to
be convex projecting in a direction away from the printing substrate 50,
i.e., towards the deflection unit 39. The pair of the projections 66 is
arranged such that the laser positioning hole 65 is positioned in between
the projections 66. Outer edges of the projections 66 are formed to fit
along the inner edge of the fitting hole 45. The pair of the projections
66 fits inside the fitting hole 45, and positions the light source unit
48 to the optical housing 35. A groove 67 is formed on each of the inner
surface of the projections 66, and the groove 67 is formed into a U-shape
in cross section, and to be flush with the inner surface of the laser
positioning hole 65.

[0085]The position of the coupling lens 54 in the light-axis direction Y
of the semiconductor laser 51 is adjusted to match the light axis of the
coupling lens 54 with the light axis of the semiconductor laser 51 and to
emit the light beam 59 as a parallel ray, and then an ultraviolet-curing
adhesive is filled in between the coupling lens 54 and respective inner
surfaces of the grooves 67 of the pair of the projections 66, so that the
coupling lens 54 is secured to the projections 66, i.e., the holder body
63.

[0086]As the projections 66 are inserted into the fitting hole 45 of the
optical housing 35, a rotating direction of the light source unit 48 is
positioned, and then the light source unit 48 is secured by press
fitting. Screws coming through the side plates 43 of the optical housing
35 are screwed into the attachment planes 68, so that the light source
unit 48 is secured to the optical housing 35.

[0087]The cylindrical lens 38 is accommodated inside the optical housing
35. The cylindrical lens 38 is provided so as to be deflected in the
sub-scanning direction Z as required. The cylindrical lens 38 receives an
incidence of the light beam 59 emitted from the light source unit 48, and
emits the light beam 59 to a reflection surface 95 of the oscillating
mirror 85 of the deflection unit 39. The cylindrical lens 38 converges
the light beam 59 in the sub-scanning direction Z on the reflection
surface 95 of the oscillating mirror 85.

[0088]As shown in FIG. 6, the deflection unit 39 includes a circuit
substrate 83, a supporting member 84, the oscillating mirror 85, and a
drive circuit (not shown) mounted on the circuit substrate 83. An example
of an electromagnetic drive system is explained below as a method of
generating torque of the oscillating mirror 85 in the first embodiment.

[0089]The circuit substrate 83 includes an insulative substrate and a
wiring pattern formed on the surface of the substrate. A control
integrated circuit and a crystal oscillator that constitute the drive
circuit of the oscillating mirror 85, a connector 86, and the like, are
mounted on the circuit substrate 83, and power from the power source and
a control signal are input and output via the connector 86.

[0090]The supporting member 84 is molded from a synthetic resin. The
supporting member 84 is positioned at a predetermined position on the
circuit substrate 83, and stands from the circuit substrate 83. The
supporting member 84 is equipped with the oscillating mirror 85. The
supporting member 84 includes a positioning unit 87, a holding hook 88,
and an edge connecter unit 89 in an integrated manner. The positioning
unit 87 positions the oscillating mirror 85 such that a twist beams 97
are to be orthogonal to the main-scanning direction X, and the reflection
surface 95 is to be inclined a predetermined angle with respect to the
main-scanning direction X, for example, 22.5 degree according to the
first embodiment. The holding hook 88 locks an outer edge of a mounting
substrate 90 of the oscillating mirror 85. The edge connecter unit 89
includes metal terminals that are arranged to come into contact with
wiring terminals 127 when the oscillating mirror 85 is mounted, the
wiring terminals 127 being formed on a side of the mounting substrate 90
of the oscillating mirror 85.

[0091]As shown in FIG. 7A, the oscillating mirror 85 is obtained as
follows: the reflection surface 95 is supported by the twist beams 97 as
a pivot; the contour is produced from a silicon substrate by etching,
which will be described later; and the etched silicon substrate is
attached onto the mounting substrate 90. A module that a pair of silicon
substrates is bonded back to back into one piece is shown in the first
embodiment.

[0092]Thus, the oscillating mirror 85 is obtained, a side of the mounting
substrate 90 is then inserted into the edge connecter unit 89, the outer
edge of the mounting substrate 90 is locked by the holding hook 88, the
both side surfaces of the mounting substrate 90 are placed along the
positioning unit 87, and then the oscillating mirror 85 is supported by
the supporting member 84. In this way, electrical wiring can be
simultaneously finished, the oscillating mirror 85 can be individually
replaced.

[0093]As shown in FIGS. 7A, 7B, 7C, and 8, the oscillating mirror 85
includes the mounting substrate 90 and a mirror unit 91. The mounting
substrate 90 is provided thereon with a mount 92 and a yoke 93. The mount
92 is a frame for mounting the mirror unit 91, and the yoke 93 is formed
to surround the mirror unit 91. The yoke 93 is attached with a pair of
permanent magnets 94. The south pole and the north pole of each of the
pair of the permanent magnets 94 oppose to each other along a direction
orthogonal to the longitudinal direction of the twist beams 97. The pair
of the permanent magnets 94 generates a magnetic field in the direction
orthogonal to the longitudinal direction of the twist beams 97.

[0094]The mirror unit 91 includes a moving part 96, the twist beams 97,
and a frame 98. The moving part 96 includes the reflection surface 95
formed on its surface and functions as an oscillator. One end of each of
the twist beams 97 is continued to each of both ends of the sub-scanning
direction Z of the moving part 96, and the twist beams 97 are placed to
stand from the both ends along the sub-scanning direction Z to form a
pivot. The frame 98 forms a support unit of which part of inner edges is
connected to the other end of each of the twist beams 97. The mirror unit
91 is formed from at least one silicon substrate, which is cut out by
etching. According to the first embodiment, the mirror unit 91 is
obtained by using a wafer called as silicon-on-insulator substrate, which
is made of two substrates 105 and 106 bonded in advance having an oxide
film in between the substrates, the substrates 105 and 106 having a
thickness of 140 micrometers and a thickness of 60 micrometers,
respectively.

[0095]The moving part 96 includes an oscillating plate 100, bracing beams
101, and a movable mirror 102. A planar coil 99 (shown in FIG. 7B) is
formed on the oscillating plate 100. The bracing beams 101 are provided
to stand from the both ends of the oscillating plate 100 in the
main-scanning direction X. The movable mirror 102 is layered on the
oscillating plate 100, and the reflection surface 95 is formed on the
movable mirror 102. The twist beams 97 can be twisted, and the moving
part 96, i.e., the reflection surface 95 is rotatable by twisting the
twist beams 97. The frame 98 includes a pair of frames 103 and 104, which
are layered.

[0096]To obtain the mirror unit 91, first of all, the substrate 105 (a
second substrate) with the thickness of 140 micrometers is etched from
the surface side of the substrate 105 according to a dry process by
plasma etching to leave the twist beams 97, the oscillating plate 100 on
which the planar coil 99 is formed, the bracing beams 101 that form bones
of the moving part 96, and the frame 103, and to pierce through the rest
of the portions up to the oxide film. The substrate 106 (a first
substrate) with the thickness of 60 micrometers is then etched by
anisotropic etching with, for example, potassium hydroxide, from the
surface side of the substrate 106 to leave the movable mirror 102 and the
frame 104 and to pierce the rest of the portions up to the oxide film.
Finally, the oxide film around the moving part 96 is removed, so that the
mirror unit 91 is formed.

[0097]It is assumed herein that the width of the twist beams 97 and the
width of the bracing beams 101 are from 40 micrometers to 60 micrometers.
As described above, to gain a large angle of twisting the moving part 96,
i.e., the reflection surface 95, it is desirable that the moment of
inertia I of the moving part 96 is small. On the other hand, the
reflection surface 95 is deformed due to the inertia force, so that the
structure of the moving part 96 is designed to be skeletal in the first
embodiment.

[0098]The reflection surface 95 is formed by depositing an aluminum thin
film on the surface of the substrate 106 that includes the surface of the
movable mirror 102. The planar coil 99 made from a copper thin film,
terminals 107 wired via the twist beams 97, and a patch for trimming are
formed on the surface of the substrate 105. Alternatively, it can be
configured that the permanent magnets 94 made as a thin film is placed on
the side of the oscillating plate 100, and the planar coil 99 is formed
on the side of the frame 104.

[0099]The mirror unit 91 is mounted onto the mount 92 in a state that the
reflection surface 95 is facing to the front. The mirror unit 91 is
configured to generate Lorentz force on each of its sides parallel to the
twist beams 97 of the planar coil 99 by passing an electric current
between the terminals 107, to twist the twist beams 97 and to generate a
torque that turns the moving part 96, i.e., the reflection surface 95,
and when the electric current is discontinued, the moving part 96 returns
to a position flush with the frame 98 due to an elastic restoring force
of the twist beams 97. Thus, the movable mirror 102 can be reciprocated
and oscillated by alternately switching the direction of the current
passing through the planar coil 99.

[0100]Additionally, in terms of time, a synchronization detecting sensor
115 arranged at a starting end of a scanning area detects the light beam
59 reflected for scanning by the reflection surface 95 of the oscillating
mirror 85 in according with a time difference between a detection signal
detected during a second-direction scanning and a detection signal
detected during a first-direction scanning, and then the angle of
twisting the reflection surface 95 is controlled to be constant. During
the time period from the detection of the light beam 59 in the
second-direction scanning until the detection of the light beam 59 in the
first-direction scanning, it is configured that a light emission of the
semiconductor laser 51 as a light emission source is inhibited.

[0101]The deflection unit 39 is accommodated in the optical housing 35,
and the light beam 59 from the cylindrical lens 38 is guided to the
reflection surface 95. The deflection unit 39 deflects the light beam 59
guided onto the reflection surface 95, and then emits the light beam 59
to an fθ lens 116 in the imaging optical system 32. When deflecting
the light beam 59, the direction of the light beam 59 is adjusted with an
adjusting screw such that the light beam comes into the central area of
the reflection surface 95 of the oscillating mirror 85, and then the
light beam 59 is deflected by the reflection surface 95, and comes into
the fθ lens 116. The deflection unit 39 is accommodated in the
optical housing 35 and blocked from outside air, so that the deflection
unit 39 is protected from change in oscillation width caused by
convection of outside air.

[0102]The light source device 31 emits the light beam 59 from the
semiconductor laser 51 of the light source unit 48 to the fθ lens
116. The light source device 31 is secured by a pair of the plate members
34 that are parallel to each other, and screws.

[0103]As shown in FIGS. 3 and 4, the imaging optical system 32 includes
the fθ lens 116 as a scanning lens, and a turn mirror 118. The
fθ lens 116 is formed into a stick of which longitudinal direction
is parallel to the longitudinal direction of the photosensitive drum 8,
attached inside the emission window 46 of the optical housing 35, and
bonded with an adhesive. The central portion of the fθ lens 116 in
the main-scanning direction X is formed into a convex shape projecting in
a direction away from the oscillating mirror 85. The fθ lens 116
lets the light beam 59 pass through, and has convergence of the light
beam 59 in the sub-scanning direction Z.

[0104]The turn mirror 118 is formed into a band plate of which
longitudinal direction is parallel to the longitudinal direction of the
photosensitive drum 8. The turn mirror 118 is arranged at an appropriate
position to guide the light beam 59 passed through the fθ lens 116
to the outer surface of the photosensitive drum 8.

[0105]According to the imaging optical system 32, the light beam 59 comes
into the fθ lens 116 from the reflection surface 95 of the
oscillating mirror 85 of the light source device 31. The light beam 59
passed through the fθ lens 116 from the light source unit 48 is
reflected by the turn mirror 118, forms an image on the photosensitive
drum 8 in a spotting manner, and forms an electrostatic latent image
based on image information.

[0106]As shown in FIG. 4, the laser writing unit 22 includes the
synchronization detecting sensor 115 for activating the semiconductor
laser 51 of the light source unit 48 in a synchronized manner. The
synchronization detecting sensor 115 receives an incidence of the light
beam 59 that is deflected by the reflection surface 95 of the oscillating
mirror 85, passes by the side of the fθ lens 116 as a scanning
lens, and then is converged by an imaging lens 122.

[0107]The synchronization detecting sensor 115 detects the light beam 59
in accordance with a time difference between a detection signal detected
during the second-direction scanning and a detection signal detected
during the first-direction scanning, and then the angle of twisting the
reflection surface 95 is controlled to be constant based on the detection
signals.

[0108]Moreover, according to the first embodiment, the synchronization
detecting sensor 115 detects an amount of light, and then a reference
value for a light-amount adjusting unit is set based on the amount of
light (signal). Therefore, the synchronization detecting sensor 115 also
functions as a light-amount detecting unit. If an amount of light is
adjusted according to the conventional auto power control, when the
position of the light source unit is deviated due to a mechanical
tolerance, the amount of light onto the scanned surface becomes
insufficient as shown in FIG. 10B. By contrast, as the synchronization
detecting sensor 115 detects an amount of light after passing though the
aperture, and the light-source activating unit as the light-amount
adjusting unit adjusts the amount of light, the amount of light onto the
scanned surface can be kept temporally constant as shown in FIG. 10A. The
term of the light-source activating unit means to include a function of a
writing control unit.

[0109]The auto power control is a method according to which a light
receiving element monitors a light output from a semiconductor laser, and
a forward current of the semiconductor laser is controlled to a desired
value based on a detection signal of a light receiving current
proportionate to the light output of the semiconductor laser.

[0110]FIGS. 11A and 11B are schematic diagrams for explaining the
oscillating mirror 85 as a light deflection unit and relevant units in
the laser writing unit 22 as the optical scanning apparatus in the image
forming apparatus 1 shown in FIG. 4.

[0111]According to the first embodiment, as shown in FIGS. 4 and 5, the
aperture 130 is provided between the semiconductor laser 51 of the light
source unit 48 and the oscillating mirror 85, and furthermore,
accommodated in the optical housing 35 and arranged between the
cylindrical lens 38 and the oscillating mirror 85.

[0112]The aperture 130 includes a body 131 formed into a flat plate, and
an opening 132 that is formed to pass through the center of the body 131.
The opening 132 is formed into a rectangle of which longitudinal
direction is in the main-scanning direction.

[0113]When the light beam 59 from the semiconductor laser 51 comes into
the reflection surface 95 of the oscillating mirror 85, the aperture 130
limits the beam width of the light beam 59 to a width appropriate to the
reflection surface 95 by letting the light beam 59 pass through the
opening 132 of the aperture 130.

[0114]Thus, the beam width of the light beam 59 can be limited by the
aperture 130 such that the irradiation position in the main-scanning
direction of the light beam 59 reliably comes into the reflection surface
95.

[0115]As shown in FIG. 11A, if the distance (an arrow) S between the
aperture 130 and the oscillating mirror 85 (strictly, the mirror unit 91)
is long, the incidence position of the light beam 59 irradiated onto the
reflection surface 95 is deviated to an end, so that a deviation occurs
on the irradiation position in the main-scanning direction of the light
beam 59 onto the reflection surface 95.

[0116]However, as shown in FIG. 11B, by making the distance S between the
aperture 130 and the oscillating mirror 85 shorter, the deviation of the
incidence position of the light beam 59 is reduced. As a result, the
deviation of the irradiation position in the main-scanning direction of
the light beam 59 onto the reflection surface 95 can be reduced, so that
the light beam 59 can be irradiated to the center of the reflection
surface 95 of the oscillating mirror 85.

[0117]According to the first embodiment, because the aperture 130 is
provided between the light source unit and the oscillating mirror 85, the
incidence position in the main-scanning direction of the light beam 59
onto the reflection surface 95 of the oscillating mirror 85 can be
adjusted without giving influence on the imaging optical system closer to
the scanned surface than the oscillating mirror 85. Accordingly, the
light beam 59 can be irradiated onto the center of the reflection surface
95 of the oscillating mirror 85 in the main-scanning direction.

[0118]Furthermore, because the aperture 130 is arranged in the vicinity of
the oscillating mirror 85 between the semiconductor laser 51 and the
oscillating mirror 85, the incidence position in the main-scanning
direction of the light beam 59 onto the reflection surface 95 of the
oscillating mirror 85 can be easily adjusted, so that the light beam 59
can be reliably irradiated onto the center of the reflection surface 95
of the oscillating mirror 85 in the main-scanning direction.

[0119]Thus, the light beam 59 can be reliably deflected at the center of
the reflection surface 95.

[0120]Moreover, because the aperture 130 is arranged between the
cylindrical lens 38 and the oscillating mirror 85, the aperture 130 can
be placed closer to the oscillating mirror 85, so that the incidence
position in the main-scanning direction of the light beam 59 onto the
reflection surface 95 of the oscillating mirror 85 can be adjusted more
effectively.

[0121]Accordingly, even if a deviation within a mounting tolerance or a
process tolerance occurs, the light beam 59 can be reliably irradiated
onto the center of the reflection surface 95 of the oscillating mirror 85
in the main-scanning direction. Thus, the light beam 59 can be reliably
deflected at the center of the reflection surface 95.

[0122]If the reflection surface 95 of the oscillating mirror 85 is waved
and deformed, a deformation in the central area of the reflection surface
95 is small, so that thickening or out-of-focus of the light beam 59 at
the imaging position can be avoided without thickening the oscillating
mirror 85, generation of a scattered light, such as a flare light due to
an eclipse of the light beam 59, is suppressed, an image in a high
quality without degradation in the image quality, such as a stain on the
background, can be created, and image processing at a high speed, in a
wide angle, and in a high quality can be achieved by reduction in the
moment of inertia due to downsizing of the oscillating mirror diameter.

[0123]Because the image forming apparatus 1 includes the laser writing
unit 22, the image is not degraded due to an eclipse of the light beam
59, the oscillating mirror 85 can be downsized, so that a high quality of
image forming, a small size of apparatus, and a high speed of image
forming can be achieved.

[0124]FIG. 12 is a cross section of the main scanning with light beam
passing. As shown in FIG. 12, the aperture 130 can be provided in a part
of the transparent member 47. According to such configuration, the
aperture 130 and the transparent member 47 can be combined into one
component, so that the number of pieces of parts can be reduced.

[0125]A second embodiment of the present invention is explained below with
reference to FIGS. 13 to 15.

[0126]FIG. 13 is a schematic diagram for explaining the oscillating mirror
85 as a light deflection unit and the aperture 130 as an aperture unit of
a laser writing unit in an image forming apparatus according to the
second embodiment of the present invention. The same components in FIG.
13 as those in the first embodiment are assigned with the same reference
numerals, and explanations of them are omitted.

[0127]According to the second embodiment, as shown in FIG. 13, the opening
132 of the body 131 of the aperture 130 is formed larger than the
reflection surface 95 of the oscillating mirror 85.

[0128]By configuring the aperture 130 in this way, the beam width of the
light beam 59 can be formed larger than the width of the oscillating
mirror 85, and the light beam 59 can be irradiated onto the whole of the
reflection surface 95 in the main-scanning direction, so that the light
beam 59 can be reliably guided to the center of the reflection surface 95
in the main-scanning direction. Thus, the light beam 59 can be reliably
deflected at the center of the light deflection unit.

[0129]According to the second embodiment, as shown in FIG. 14A, the
reflection surface 95 of the oscillating mirror 85 is configured not to
have curvature on the edges of its both ends in the main-scanning
direction (to be substantially straight in the sub-scanning direction).
According to the second embodiment, because the beam width of the light
beam 59 is larger than the width of the oscillating mirror 85, a shape of
the light beam 59 to be cut is determined in accordance with the shape of
the oscillating mirror 85.

[0130]In other words, the beam width of the light beam 59 in the
main-scanning direction is limited by the reflection surface 95.

[0131]In this way, the shape of the light beam 59 after deflection is
formed into a rectangle, and degradation of wavefront aberration when
imaging on the scanned surface is reduced not to degrade the beam spot
diameter.

[0132]When it is advantageous in terms of processing to have a curvature
on the edges of the both ends of the moving part 96 of the oscillating
mirror 85 in the main-scanning direction, the reflection surface 95
without curvature on the edges of its both ends in the main-scanning
direction can be provided on the moving part 96, as shown in FIG. 14B. In
other words, the width of the reflection surface 95 in the main-scanning
direction can be formed smaller than the width of the light deflection
unit (strictly, the moving part 96) in the main-scanning direction.

[0133]According to the second embodiment, because a beam width to be cut
is determined in accordance with the diameter on the reflection surface
95 in the main-scanning direction, the beam width to be cut is changed in
accordance with an image height to be scanned, as shown in FIGS. 15A and
15B. As shown in FIG. 15A, suppose a is the beam width of an incident
beam cut at the reflection surface 95 when scanning an image height close
to the light source (hereinafter, "plus image height"), while b is the
beam width of the incident beam cut at the reflection surface 95 when
scanning an image height far from the light source (hereinafter, "minus
image height") as shown in FIG. 15B, a is larger than b.

[0134]In other words, the amount of light on the scanned surface when
scanning the plus image height is more than that when scanning the minus
image height, and the change in the amount of light is monotone
decreasing.

[0135]For this reason, according to the second embodiment, the transparent
member 47 is provided with a shading property as shown in FIGS. 16 and
17. A graph is shown in FIG. 16, in which the amount of light for
scanning an image height of 150 millimeters at the highest transparency
is presented as the reference for amounts of light for all image heights.

[0136]The shading property is set to prevent the irregularity of the
amount of light on the scanned surface by arranging the amount of light
for the plus image height less than the amount of light of the minus
image height to cancel the irregularity of the amount of light on the
scanned surface among image heights.

[0137]In other words, as shown in FIG. 17, the material of the transparent
member 47 is adjusted such that the transparency on the plus image height
side (incidence side) is low, and the transparency on the minus image
height side (opposite side to the incidence) is high. Accordingly, the
transparent member 47 is to be a light-amount adjusting unit that adjusts
along the main-scanning direction the amount of light of a light beam
deflected by the light deflection unit.

[0138]Alternatively, according to a third embodiment of the present
invention, irregularity of the amount of light can be suppressed by
adjusting along the main-scanning direction an integral amount of light
per dot, as shown in FIGS. 18A and 18B. FIG. 18A is a graph when the
intensity of a light beam for an image height on the opposite side to the
incidence (minus image height) is set higher than the intensity of a
light beam for an image height on the incidence side (plus image height).
The relation between image heights and respective intensities of the
light beam is similar to the shading property shown in FIG. 16. The light
pulse width for forming a dot is equally set across all of the image
heights. The responsible unit that adjusts the integral amount of light
per dot is the light-source activating unit.

[0139]FIG. 18B is a graph when the light pulse width for forming a dot of
an image height on the opposite side to the incidence (minus image
height) is set larger than the light pulse width for forming a dot of an
image height on the incidence side (plus image height). The relation
between image heights and respective light pulse widths is similar to the
shading property shown in FIG. 16. The light beam intensity is equally
set across all of the image heights.

[0140]In this way, irregularity of the amount of light can be suppressed
by adjusting the integral amount of light per dot in accordance with an
image height.

[0141]A fourth embodiment of the present invention is explained below with
reference to FIG. 19.

[0142]According to the optical scanning apparatus of the image forming
apparatus 1 in the embodiments described above, the outer surface of one
unit of the photosensitive drum 8 is scanned by the oscillating mirror 85
with the light beam 59 from one unit of the light source unit 48.

[0143]However, the optical scanning apparatus according to the fourth
embodiment of the present invention can be applied to a multicolor image
forming apparatus for more than one color, or a full-color image forming
apparatus, as shown in FIG. 19. FIG. 19 is a schematic diagram for
explaining a modification of the laser writing unit 22 shown in FIG. 2.

[0144]The same components in FIG. 19 as those in the first embodiment are
assigned with the same reference numerals, and explanations of them are
omitted.

[0145]According to an example shown in FIG. 19, four light beams 59, 60,
61, and 62 from a plurality of light source units 48a and 48b of a laser
writing unit 22' as an optical scanning apparatus of an image forming
apparatus are guided to a plurality of photosensitive drums 8Y, 8M, 8C,
and 8K.

[0146]The laser writing unit 22' includes a light source device 31' and an
imaging optical system 32'. The light source device 31' includes the
optical housing 35, the light source units 48a and 48b, an incident
mirror 37, the cylindrical lens 38 as an image forming lens, and the
deflection unit 39.

[0147]Each of the light source units 48a and 48b includes a pair of
semiconductor lasers (not shown), and the semiconductor lasers emit the
light beams 59, 60, 61, and 62 corresponding to the photosensitive drums
8Y, 8M, 8C, and 8K, respectively.

[0148]Each two of the semiconductor lasers are arranged in each of the
light source units 48a and 48b such that the light beams 59, 60, 61, and
62 make an angle of 2.5 degree, and cross each other on the reflection
surface 95 of the oscillating mirror 85.

[0150]The incident mirror 37 emits the light beams 59, 60, 61, and 62 in a
state where the light beams 59, 60, 61, and 62 from the semiconductor
lasers are vertically aligned in a line (aligned along the sub-scanning
direction Z), maintaining intervals in the sub-scanning direction Z
between them.

[0151]The imaging optical system 32' includes the fθ lens 116, a
plurality of toroidal lenses 117Y, 117M, 117C, and 117K, and the turn
mirrors 118. The fθ lens 116 is arranged such that the longitudinal
direction of the fθ lens 116 is in parallel with the longitudinal
direction of the photosensitive drums 8Y, 8M, 8C, and 8K.

[0152]The toroidal lenses 117Y, 117M, 117C, and 117K are provided
correspondingly to the photosensitive drums 8Y, 8M, 8C, and 8K,
respectively, and each formed into a stick shape of which longitudinal
direction is in parallel with the longitudinal direction of the
photosensitive drums 8Y, 8M, 8C, and 8K. Each of the toroidal lenses
117Y, 117M, 117C, and 117K lets pass only one corresponding beam of the
light beams 59, 60, 61, and 62 each of which scans the outer surface of
one corresponding drum of the photosensitive drums 8Y, 8M, 8C, and 8K.

[0153]The turn mirrors 118 are each formed into a band plate of which
longitudinal direction is in parallel with the longitudinal direction of
the photosensitive drums 8Y, 8M, 8C, and 8K, and arranged at respective
appropriate positions to guide the light beams 59, 60, 61, and 62 passed
through the fθ lens 116 to the outer surfaces of the photosensitive
drums 8Y, 8M, 8C, and 8K via the toroidal lenses 117Y, 117M, 117C, and
117K, respectively.

[0154]According to the laser writing unit 22' configured as described
above, the incident mirror 37 emits the light beams 59, 60, 61, and 62
from the light source units 48a and 48b of the light source device 31' in
a state that the light beams 59, 60, 61, and 62 are aligned along the
sub-scanning direction Z and maintaining intervals between them. The
light beams 59, 60, 61, and 62 are passed through the cylindrical lens 38
and emitted into parallel rays. The aperture 130 limits respective beam
widths of the light beams 59, 60, 61, and 62, and leads the light beams
59, 60, 61, and 62 from the light source units 48a and 48b to grazing
incidence onto the oscillating mirror 85 at different angles in the
sub-scanning direction Z. Accordingly, the light beams 59, 60, 61, and 62
from the light source units 48a and 48b are collectively deflected and
reflected, so that the fθ lens 116 as a scan lens receives
incidences of the light beams 59, 60, 61, and 62 deflected and reflected
by the reflection surface 95.

[0155]The light beams 59, 60, 61, and 62 passed through the fθ lens
116 are separated into individual colors by the toroidal lenses 117Y,
117M, 117C, and 117K, and reflected by respective mirrors of the turn
mirrors 118 corresponding to the photosensitive drums 8Y, 8M, 8C, and 8K,
and form an image into a spot and create an electrostatic latent image
based on image information onto the photosensitive drums 8Y, 8M, 8C, and
8K, respectively.

[0156]According to the embodiments described above, the optical scanning
apparatus includes the oscillating mirror 85 as a light deflection unit.
However, according to the present invention, a polygon mirror that is
generally used in a conventional optical scanning apparatus can be used,
so that the light deflection unit according to the present invention is
not limited to an oscillating mirror.

[0157]The embodiments described above only describe typical forms
according to the present invention, and the present invention is not
limited to the above embodiments. In other words, the embodiments can be
implemented in various modifications within a scope not departing from
the gist of the present invention.

[0158]According to the embodiments of the present invention, change of the
incidence position of the light beam onto the oscillating mirror due to a
deviation within a mounting tolerance and an assembling tolerance can be
suppressed. Moreover, the aperture unit can be arranged closer to the
reflection surface, and the light beam can be guided to the vicinity of
the center of the oscillating mirror more efficiently. As a result, the
optical scanning apparatus can reduce degradation of the beam spot
diameter of the light beam in the main-scanning direction and image
surface curvature, which are caused by a deviation of local
light-convergence power due to dynamic surface deformation of the
oscillating mirror, so that the beam spot diameter in the main-scanning
direction can be kept uniform in size across the scanned surface.

[0159]Furthermore, the light beam can be irradiated all over the
oscillating mirror, so that such irradiation has the same effect as the
light beam is irradiated to the vicinity of the center of the oscillating
mirror without arranging the aperture unit.

[0160]Moreover, when the optical scanning apparatus is configured to
irradiate the light beam all over the oscillating mirror, degradation of
wavefront aberration of the deflected light beam can be reduced, so that
degradation of the beam spot diameter on the scanned surface can be
suppressed.

[0161]Furthermore, also when it is difficult in terms of processing to
eliminate a curvature on the edges of the both ends of the reflection
surface in the main-scanning direction, degradation of wavefront
aberration of the deflected light beam can be reduced, so that
degradation of the beam spot diameter on the scanned surface can be
suppressed.

[0162]Moreover, the number of items of parts can be reduced by using the
transparent member for common use as the aperture unit, so that the
optical scanning apparatus can be provided at a low cost.

[0163]Furthermore, irregularity of the amount of light among image heights
can be reduced on the scanned surface, so that an image in a high quality
that irregularity in density is suppressed can be formed.

[0164]Moreover, because the amount of light after deflection and
reflection is detected, and the reference value for the light-amount
adjusting unit is set based on the signal of the detected amount of
light, when the position of the light source changes due to a deviation
within a tolerance, the scanned surface can be exposed with an reliable
amount of light, so that image forming in a high quality without
irregularity in density and with no stain on the background can be
achieved.

[0165]Furthermore, as the beam spot diameter and the amount of light are
uniformly kept across the whole scanned surface by adjusting the
incidence position of the light beam onto the light reflection surface,
the image forming apparatus that can form an image in a good quality can
be provided.

[0166]Although the invention has been described with respect to specific
embodiments for a complete and clear disclosure, the appended claims are
not to be thus limited but are to be construed as embodying all
modifications and alternative constructions that may occur to one skilled
in the art that fairly fall within the basic teaching herein set forth.